The continuing depletion of fossil and nuclear fuels may be one of the most significant long term problems facing the world. There is condiderable disagreement regarding the size of the depletable fossil and nuclear fuel resources. Thus, increasing interest is now centered about renewable energy resources such as solar energy.
The collection and concentration of solar energy is an ancient art and accordingly over the span of many centuries numerous solar energy collector and concentrator systems have been devised. Until relatively recently the total solar energy collection and concentration capability of such systems was relatively small and generally confined to heating systems or endothermic industrial processes requiring relatively low levels of energy input.
These prior art systems generally can be characterized as fixed or movable arrays of reflectors wherein the array elements may be fixed at a given azimuth or configured to comprise heliostat elements that include means for adjustment that enables automatic or manual tracking of the sun to maximize solar energy collection and concentration.
The movable arrays are generally carried by a sun tracking support that is moved through a predetermined orbit to track around an axially disposed energy receiver, such as a furnace, boiler, vaporizer, etc. As will be readily appreciated, the energy collection capability, or capacity, of such movable arrays is as a practical matter rather limited in view of the engineering problems attendant the movement of relatively large arrays.
Thus, more recent attempts to collect relatively large amounts of solar energy for concentration and utilization for electrical power generation, industrial uses, and the like, have been centered about the utilization of a solar array collector system herein referred to as a distributed field heliostat array. As is well known, the collector arrays presently generally used in the United States Energy Research and Development Administration Solar Thermal Conversion Central Power Projects utilize the distributed field heliostat array that distributes numerous heliostats over a field, commonly a very large tract of land, and wherein the substantial number of heliostats are each separately supported on pedestals, or foundations, in the distributed field. Since large expanses of collector surfaces are expensive, and since land values in industrialized areas are generally very high, a primary factor in reducing the capital investment directly attributable to the development of solar energy for industry is the efficient use of collector surface and land.
In addition, an energy receiver generally associated with a distributed field array comprises an energy receiver means mounted on a tower and wherein the energy receiver requires an entry port for collected and concentrated solar energy. In such an installation the entry port has a relatively wide aperture angle and to increase the energy input to the central receiver the spacing between the collector array and the central receiver must increase if the central receiver entry port aperture angle remains constant. In the distributed field collector array systems this requires vertical separation between the collector array and the central energy receiver.
The system described, which has been referred to as a power tower system in a presently proposed project for 100 MWe being developed as a booster system for an existing electrical generation plant, contemplates acres of mirrors in a field to reflect the sun's heat to a water boiler stop a 1000-foot tower. More specifically the proposed project envisions the utilization of at least 170 acres of land to accommodate the distributed field of collectors, or heliostats. Alternatively the same project proposes to attempt to utilize three towers each 430 feed high instead of a single 1000-foot tower.
It will be appreciated that central receiver towers of the aforementioned height introduce possible air space and construction problems. Further, the energy concentration ratio for a given collector array is partially a function of shading of one collector element by another due to sun position or angle off the axis of the central receiver energy collector aperture and also shading of collector elements by the tower and boiler structure. Shading in the distributed field collector, or heliostat, array systems is a function of both solar declination and the time of day.
An open sky collector system is exempified by U.S. Pat. No. 3,118,437, Jan. 21, 1964, which also appears to be closely related structurally to a solar energy collection system at Odeillo, France. Such systems are considered to be representative of the prior art systems that attempt to concentrate, as well as collect solar energy from the previously discussed distributed field heliostat arrays. These systems are characterized by the utilization of duplex reflector systems which it will be appreciated are not generally suitable for concentration of solar energy at a high order as is required for cost effective solar energy utilization for power generation, and the like.
As the description of the present invention proceeds additional distinguishing aspects and advantages will be set forth with reference to the distributed field array system discussed.